Turning Sunlight into Synthetic Fuel

Sandia researchers say the aim is to build a reactor about the size of an oil drum. Such a generator could transform 22 kilograms of carbon dioxide and 18 kg of water into roughly 9.5 liters of liquid fuel each day, but commercialization is 15-20 years away.

Published: 08-Jan-2008

At first blush, you might lump claims about a machine that supposedly turns sunshine, air, and water into fuel in the same category as e-mails insisting that someone in Nigeria will pay you handsomely to help free up a large sum of money. But researchers at the U.S. Department of Energy’s Sandia National Laboratories, in Albuquerque, say they have created a device that can break water into hydrogen and oxygen using sunlight, or in a another reaction convert carbon dioxide, to carbon monoxide that combines with hydrogen to make hydrocarbons such as methanol, ethanol, and even gasoline or diesel fuel. The technology holds the promise of using the same resources as biomass-to-fuel schemes but with potentially greater efficiency, according to the researchers.

The machine, called the Counter Rotating Ring Receiver Reactor Recuperator, or CR5, is essentially a stack of rings or disks, each outfitted with a dozen fins around its circumference that are constructed of a reactive metal oxide (rust to you and me) in a matrix of material that can withstand high temperatures. As a ring rotates, the reactive material passes into a chamber irradiated by a solar collector where temperatures exceed 1500 °C. This is hot enough to trigger a reaction that liberates oxygen from the rust, changing the metal’s chemical structure at the same time. The ring rotates the metal oxide (at roughly 1 revolution per minute) 180 degrees to a reaction chamber where, at relatively cool temperatures (around 1000 °C), the scorched rust is exposed to superheated steam. The metal oxide and water react in a way that effectively strips the oxygen from the water and restores the rust to its original form, yielding free hydrogen in the process. Then the cycle begins again.

In another CR5 stack, carbon dioxide is split into carbon monoxide and oxygen in the solar chamber. The carbon monoxide could then be used to make synthetic hydrocarbon fuels by combining it with hydrogen from the first CR5 stack, using any of a number of commercial processes.

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